1. Field of the Invention
The present invention is related to an organic electroluminescence element and a method of manufacturing the same. More particularly, the present invention is related to a method of manufacturing an organic electroluminescence display device including a wet process, wherein repellent phenomenon or unevenness in each layer of an organic light emitting medium layer does not appear, wherein each pixel emits light uniformly, and wherein the organic electroluminescence display device has a long life time and high efficiency without a dark spot (non-light emitting spot) and leakage of electric current.
2. Description of the Related Art
An organic electroluminescent element has an organic light emitting layer presenting an electroluminescent phenomenon between an anode and a cathode. When a voltage is applied between the electrodes, a hole and electrons are injected into an organic light emitting layer. Then, the hole and electrons recombine in the organic light emitting layer and the organic light emitting layer emits light. In other words, an organic electroluminescent element is a self-luminous element.
From a purpose to improve light emitting efficiency, a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer are further installed appropriately. A hole injection layer and a hole transport layer are installed between an anode and an organic light emitting layer. An electron transport layer and an electron injection layer are installed between an organic light emitting layer and a cathode. An organic light emitting layer, a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer can be referred to as an organic light emitting medium layer.
Light emitting efficiency and luminance depend on the film thickness of each layer, therefore the film thickness should be around 100 nm. More particularly, patterning of high accuracy should be formed to make a display panel.
Low molecular materials and polymeric materials are examples of organic light emitting medium materials that can be used to form an organic light emitting medium layer. Generally, as for low molecular materials, a thin film is formed by vacuum evaporation. A mask with minute patterns is used at this time, and patterns are formed. In a manufacturing method which uses vacuum processing such as vacuum evaporation, upsizing of a substrate can diminish the accuracy of the patterning. In addition, due to layering in vacuum, throughput is bad.
Thus the method whereby a polymer material is dissolved in a solvent, and ink is made has been tried recently. This ink is used, and thin film is formed by a wet coating method.
A layer structure in the case where an organic light emitting medium layer is formed by wet coating method using a solution of polymer materials is generally a two-layer structure in which a hole transport layer and an organic light emitting layer are laminated from the anode side.
At this time, in the organic light emitting layer, it is necessary for the organic light emitting inks including organic light emitting materials of red (R) green (G) and blue (B) in a solvent to be applied independently in order to form a color panel.
Examples of methods for forming an organic light emitting medium layer by wet processes include mainly spin coat, die coat, inkjet and print.
Patterning can not be performed only by spin coat or die coat among the above-mentioned methods. Patterning can be performed only by inkjet or print; however ink materials, ink solvent and viscosity are limited, therefore it is difficult to perform stable patterning in inkjet or print. The more high-definition progresses, the more a color ink mixed with another color ink in an adjacent pixel becomes prominent, thereby it becomes more difficult to perform patterning stably and precisely.
In addition, in a case where a color panel is manufactured by the method of not only wet process but also other process, patterning of an electrode is necessary, therefore there is a problem that a leakage current occurs since an electric field concentrates on an end of a patterned electrode.
Accordingly, in a case where patterning is performed, a partition wall pattern as a convex shaped sectioning member is provided in order to solve these problems. A partition wall pattern is formed by a conventional photography process comprising applying a photoresist to a substrate, exposure and development.
Generally a photoresist includes a little of a surface-active agent in order to improve coating performance and film formation performance and in order to prevent unevenness. These surface-active agents are generally hydrophobic. Particularly, the surface-active agents bleed out too much on a substrate in a heating process, thereby an ink repellent phenomenon or a void in a pixel easily occurs when a water type hole transport ink is applied to a substrate.
In addition, in a hardening process, a hydrophilic functional group frequently becomes a bridge formation point, thereby there is a problem that the hydrophilic character of a resin after hardening is lowered.
On the other hand, a hole transport layer is generally applied on the entire area contributing to imaging of an organic electroluminescent display panel without patterning the hole transport layer. A hole transport layer is formed by coating methods such as a spin coat method and a die coat method. Generally a hole transport layer is a thin film with a thickness equal to or less than 100 nm. Therefore, an electric current is easier to spread in a depthwise direction of a hole transport layer than a transverse direction of a hole transport layer. Therefore, it is said that leakage current to the outside of a picture element is small if patterning of an electrode is performed. In the above-mentioned coating method, patterning for every pixel is not necessary, however a hole transport layer is formed on a place where an adhesive for sealing is to be applied and a place where a driver chip is to be equipped, thereby it is necessary to wipe and remove an unnecessary part of a hole transport layer after the hole transport layer is applied. It was very important to form a pattern of not only an organic light emitting layer but also a hole transport layer only on a pixel electrode in order to shorten the manufacturing process and reduce manufacturing costs.
Particularly, in a hole transport material, a water type ink is often used. Therefore, in order to be able to form a flat transport layer, it was necessary to make a partition wall hydrophilic by irradiation with an ultra violet ray, oxygen plasma or the like before forming a hole transport layer as well as to add a solvent or another surface-active agent to a hole transport ink in order to reduce surface tension of the hole transport ink.
Making a partition wall hydrophilic by irradiation with an ultra violet ray is very effective; however there were problems in that a surface of a partition wall which contacted an hole transport ink was not made sufficiently hydrophilic or a surface of a partition wall was not hydrophilic for long time. In addition, there was a problem that, even if the solvent used to reduce a surface tension was soluble in water, an ink composition became unstable, thereby precipitation and unevenness easily occurred since it was difficult to dry the ink uniformly.
In addition, since a surface-active agent generally has low volatile properties, a surface-active agent remains in a hole transport layer after drying, thereby it is pointed out that a surface-active agent badly effects light emitting performance and the life time of an organic EL device. The number of the kinds of usable inks which solve the above-mentioned problem is limited and thereby, a formula for an ink is difficult to produce.
If the surface tension of a partition wall is adjusted to fit to a hole transport material, an organic light emitting layer becomes uneven at a part near the partition wall since the solvent in the organic light emitting layer is different from the solvent in the hole transport material. This phenomenon causes a problem such as a non-lighting spot or a non uniform emitting light. That is, both a flat hole transport layer and a flat organic light emitting layer can not be obtained simultaneously.
Further, even in a case where a layer is formed on the entire surface or a layer is formed in a pattern shape by a dry process when an inorganic material is used for a hole transport layer, in a panel including a partition wall formed by using an organic material, an inorganic material of the hole transport layer is deteriorated by an out gas from the partition wall. This causes a non-lighting spot or a leakage of electric current, and life time becomes shorter and efficiency is reduced.
On the other hand, an organic type solvent is used for an organic light emitting layer, therefore a surface-active agent or a residual solvent included in a partition wall, and a material of a partition wall may liquate out into an organic light emitting layer. This phenomenon may have harmful effects on the light emitting efficiency or life time.
Recently, a technology in order to improve fabrication yield is disclosed whereby a first organic material and a second organic material are layered, thereafter both the formed layers are simultaneously exposed and developed to produce a first insulating layer and a second insulating layer as partition walls. (See patent document 1.)
However, in a panel including a partition wall formed by using an organic material, an light emitting molecule of an organic light emitting layer is deteriorated by an out gas from the partition wall. This causes a non-lighting spot or a leakage of electric current, and life time and efficiency is reduced. In addition, an impurity such as a residual solvent in a partition wall or a material of a partition wall liquate out by a solvent included in the organic light emitting medium layer, thereby light emitting efficiency and life time is reduced.    [patent document 1] Japanese Laid Open Patent No. 2004-319119    [patent document 2] Japanese Patent No. 3328297